Hundreds of thousands gathered Thursday to mark the anniversary of Iran's Islamic revolution, as authorities clamped down on scattered protests. President Mahmoud Ahmadinejad hailed new success with uranium enrichment.

Demonstrators wave Iranian flags, as a picture of supreme leader Ayatollah Ali Khamenei, is held at center, during a rally commemorating the anniversary of the 1979 Islamic Revolution, at the Azadi (freedom) Square in Tehran on Thursday.

TEHRAN, Iran (AP)  Hundreds of thousands of government supporters massed Thursday in central Tehran to mark the anniversary of the revolution that created Iran's Islamic republic, while a heavy security force that fanned across the city moved quickly to snuff out counterprotests by the opposition.

Police clashed with protesters in several sites around Tehran, firing tear gas to disperse them and paintballs to mark them for arrest, opposition websites reported. Dozens of hard-liners with batons and pepper spray attacked the convoy of a senior opposition leader, Mahdi Karroubi, smashing his car windows and forcing him to turn back as he tried to join the protests, his son Hossein Karroubi told The Associated Press.

The celebrations were an opportunity for Iran's clerical regime to tout its power in the face of the opposition movement, which has persisted in holding mass street protests since disputed presidential elections in June in defiance of a fierce monthslong crackdown.

The security clampdown appeared to have succeeded in preventing a major opposition turnout. Their numbers were not immediately known — but opposition websites spoke of groups of protesters in the hundreds, compared to thousands in past demonstrations.

One protester told The Associated Press she had tried to join the demonstrations but soon left in disappointment. "There were 300 of us, maximum 500. Against 10,000 people," she told an AP reporter outside Iran.

"It means they won and we lost. They defeated us. They were able to gather so many people," she said. "But this doesn't mean we have been defeated for good. It's a defeat for now, today. We need time to regroup." She spoke on condition of anonymity for fear of retaliation by authorities, who have jailed protesters for talking to foreign media.

Foreign media in Iran were banned from covering the pro-reform protests. Tehran residents also reported Internet speeds dropping dramatically and e-mail services such as Gmail being blocked in a common government tactic to foil opposition attempts to organize.

Heavy numbers of riot police, members of the Revolutionary Guard and Basij militiamen deployed at key squares and major avenues in the capital to prevent protests from marring the annual mass rallies for the revolution's anniversary.

State television showed images of thousands upon thousands carrying often identical banners marching along the city's broad avenues toward the central Azadi, or Freedom, Square. There, the massive crowds waved Iranian flags and carried pictures of Ayatollah Ruhollah Khomeini, the founder of the Islamic state, and his successor as supreme leader, Ayatollah Ali Khamenei.

In a nationally televised address in the square, Iranian President Mahmoud Ahmadinejad proclaimed that Iran has produced its first batch of uranium enriched to a higher level, saying his country will not be bullied by the West into curtailing its nuclear program a day after the U.S. imposed new sanctions.

"The first package of 20% fuel was produced and provided to the scientists," he said, reiterating that Iran was now a "nuclear state." He did not specify how much uranium had been enriched.

Iran announced on Tuesday that it was starting for the first time to further enrich uranium from around 3% purity to 20% purity, bringing sharp criticism from the United States and its allies, which accuse Tehran of seeking to develop a nuclear weapon.

Tehran, which denies seeking to build a bomb, has said it wants to further enrich the uranium — which is still substantially below the 90% plus level needed for a weapon — to fuel a research reactor for medical isotopes.

For days ahead of the anniversary celebrations, anti-government websites and blogs have called for a major turnout in counterprotests and urged marches to display green emblems or clothes, the signature color of the opposition.

Security forces fired tear gas to disperse a group of protesters who were trying to march toward Azadi Square as they chanted "death to the dictator," the opposition website Rahesabz said, reporting an unknown number of arrests. Police and Basijis on motorbikes swept toward central Tehran, where protesters and security forces clashed in several locations, it and other opposition websites reported.

Riot police fired paint-filled balls after several hundred protesters began to chant opposition slogans in Sadeqieh Square, about a half-mile (one kilometer) from the huge pro-government gathering, witnesses said.

Witnesses said there were no apparent injuries among the protesters.

Security forces also briefly detained the granddaughter of Ayatollah Ruhollah Khomeini, the architect of the 1979 Islamic Revolution, and her husband, who are both senior pro-reform politicians, according to the couple's son, Ali.

The granddaughter, Zahra Eshraghi, and her husband Mohammad Reza Khatami, who is the brother of a former pro-reform president, were held for less than an hour before being released, their son told the AP.

The witnesses spoke on condition of anonymity for fear of retribution from authorities. Foreign media were only allowed to cover the ceremonies in the square and the speech by Ahmadinejad, with photographers bused to the site and then away. There is an explicit ban on covering opposition protests.

Iranian authorities again tried to squeeze off text messaging and Web links in attempts to cripple protest organizers. Internet service was sharply slowed, mobile phone service widely cut and there were repeated disruptions in popular instant messaging services such as Google chat, though some were sporadically accessible.

Many Internet users said they could not log into their Gmail account, Google's e-mail service, since last week.

"We have heard from users in Iran that they are having trouble accessing Gmail," Google said in a statement. "We can confirm a sharp drop in traffic and we have looked at our own networks and found that they are working properly."

Opposition members went on roof tops late Wednesday and shouted Allah-u-Akbar ("God is greatest") in protest — echoing similar cries after the disputed June election as well as anti-shah protests more than three decades ago.

The opposition claims that Ahmadinejad's victory in the June 12 election was fraudulent and that the true winner was pro-reform leader Mir Hossein Mousavi. Hundreds of thousands marched in the streets against the government in the weeks after the vote, prompting a massive wave of arrests.

Nevertheless, the opposition has succeeded in continuing to hold regular protests, often timing them to coincide with days of important political or religious significance in attempts to embarrass authorities. The tone of the rallies has shifted from outrage over alleged fraud in Ahmadinejad's re-election to wider calls against the entire Islamic system, including Khamenei.

Tensions have mounted further since the last large-scale marches, in late December, which brought the most violent battles with security riots in months. At least eight people were killed in clashes between protesters and police, and security forces have intensified arrests in the weeks since.

In January, two people who were put on trial alongside opposition politicians and protesters were executed for allegedly plotting to overthrow the state. Authorities have announced that nine other opposition supporters have also been sentenced to death — a move many believe was aimed at intimidating protesters.

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